The present invention generally relates to battery backup systems and in particular to a system and method of selecting a bias resistor and a test load for battery backup systems employed in modules that have load expansion capabilities. The present invention has particular applicability to industrial control modules utilizing volatile memory.
Backup power sources are utilized in many electronic devices for providing alternate power in the event that the main power to the electronic device is temporarily or permanently removed, such as in the case of a periodic shutdown or a power loss. Typically, these backup power sources (e.g., batteries or the like) are periodically assessed to determine whether or not the backup power source has enough charge to provide power for the load it is designed to operate. Testing a backup power source can be accomplished by temporarily connecting the backup power source to a resistor sized to correspond to a worse case current draw that the actual load can experience. In addition, switches are used, for example such as a PNP transistor switch, to connect the backup power source voltage to the load being driven. The PNP transistor switch includes a base lead bias resistor that is also sized to correspond to the actual load being driven at the collector lead for a worse case condition.
A problem evolves in the event that the module employed that utilizes the backup power source is expandable to drive additional loads. In these situations, the backup source test load and the bias resistor are set for currents that ensure the ability to drive the maximum load that may be provided utilizing a worse case expansion with all modules operating in a worse case condition. However, in many cases a user may utilize a much smaller expansion load or no expansion load at all. The result is that the backup power source life is reduced as a result of a larger than necessary bias current and a larger than necessary test load current. Furthermore, the service life of the backup power source is reduced due to the fact that the backup power source voltage is reduced more at the higher test load, causing it to reach a service warning level faster than it would at a lower test load current.
For example, if the backup power source is a primary lithium battery cell and the load is a volatile memory, the PNP transistor switch acts as a reverse current blocker/limiter to prevent the primary lithium cell from being charged. Traditionally, both the backup power test load and PNP bias current would be set to accommodate the maximum amount of memory to be supported. However most memory expansion modules have a wide range of memory options possible, for example, 2 MB may use 2 mA of battery current, 4 MB may use 4 mA of battery current and 8 MB may use 8 ma of battery current. If a bias resistor and a test load resistor are set for 8ma for a 2MB memory, then battery service life for the 2 MB memory option suffers by 8 to 28%, depending on temperature. This causes many applications with the lower memory options to lose significantly more battery service life than necessary to enable a small percentage of applications to install a 8 MB option module.
Accordingly there is an unmet need in the art to provide expandable systems employing backup power sources with appropriate test loads for any given configuration. Additionally, there is an unmet need in the art to provide expandable systems employing backup power sources utilizing bias resistors to drive power switches with appropriate bias resistances for any given configuration.
The present invention provides for a system and method for selecting a test load and/or bias resistor utilized in an expandable load system employing a backup power source. The test load and/or bias resistor is selected based on the actual load present in the expandable system. For example, the test load can be selected (e.g. sized) to have a load substantially equivalent to an actual load of the system operating in a worse case condition. A service warning signal can then be initiated if the backup power source voltage falls below a predetermined voltage level during a test cycle. Alternatively, the test load can be sized to draw current at a proportionate level of the actual load and the predetermined voltage level increased. Regardless of the selected test load level and corresponding predetermined voltage level, the test load is selected to be sized based on the actual load of the system, such that the test load provides adequate determination of the backup power source to provide current drawn by the actual load in a worse case condition.
During normal operation, the backup power source is periodically tested utilizing the test load and comparing voltage of the backup power source under test load to a voltage warning signal level. If an expansion load is not connected in the system, a test load is provided that is sized based on the actual load of a local load under worse case conditions. If an expansion load is connected to the local load the test load is adjusted, so that the test load remains based on the actual load, which now becomes a combination of the local load and the expansion load. The adjustment of the test load can be provided by utilizing hardware (e.g., placing a resistor in parallel with test load) or via hardware (e.g., reading an ID of the expansion load and setting the test load accordingly).
Additionally, a bias resistance can be provided for driving a switch (e.g. a PNP transistor) that allows power to be provided to an actual load during power loss mode by the backup power source. If an expansion load is not connected in the system, the bias resistance is selected based on the collector current necessary to provide power to a local load under a worse case condition. If an expansion load is connected to the local load, the bias resistance is adjusted to correspond to a bias resistance necessary to provide the current for the actual load of the local load and the expansion load under a worse case condition. The adjustment of the bias resistance can be provided by utilizing hardware (e.g., placing a resistor in parallel with a local bias resistor) or via software (e.g., reading an ID of the expansion load and setting the bias resistance accordingly).
In one aspect of the invention, a system and method is provided for sizing a test load and/or bias resistor utilized in a system having expandable volatile memory and employing a backup battery. The test load and/or bias resistor are sized based on the actual memory present in the expandable system. The backup battery is periodically tested during normal operation of the expandable memory system utilizing the test load. The test load is sized based on the actual load of a local memory under worse case conditions. If an expansion memory is connected to the local memory, the test load is adjusted by placing a second test load across the first test load, so that the test load remains based on the actual load, which in this case is the load of the local memory and the expansion memory. Additionally, a base lead bias resistance is provided for driving a PNP transistor that allows power to be provided to the local memory during power loss mode. The base lead bias resistance is selected based on the collector current necessary to provide power to a local memory under a worse case condition. If an expansion memory is connected to the local memory, the base lead bias resistance is adjusted to correspond to a base lead bias resistance necessary to provide the collector current for the actual load of the local memory and the expansion memory under worse case conditions.
The following description and the annexed drawings set forth in detail certain illustrative aspects of the invention. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.